Methods and apparatus for extended merge mode with adaptive grid size in video coding

ABSTRACT

A method and apparatus for searching for merge candidates for inter-prediction coding of a video sequence, including performing a comparison between a size of a current CU and a threshold size, based on a result of the comparison, changing a size of a search grid used to search for the merge candidates in order to construct a merge candidate list.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to U.S.Application No. 62/680,497, filed on Jun. 4, 2018, in the United StatesPatent & Trademark Office, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD

This disclosure is directed to the next-generation video codingtechnologies beyond High Efficiency Video Coding (HEVC), such as, forexample, Versatile Video Coding (VVC). More specifically, the presentdisclosure is directed to several methods for inter-picture predictioncoding, for example for merge mode. Extra spatial or temporal mergecandidates are inserted into the merge candidates list. The constructionof merge candidate list is modified. New signaling scheme of the mergeindex is also discussed.

BACKGROUND

In HEVC, a merge mode for Inter-picture prediction is introduced. Amerge candidate list of candidate motion parameters from neighboringblocks is constructed. Then an index is signaled which identifies thecandidates to be used. Merge mode also allows for temporal prediction byincluding into the list a candidate obtained from previously codedpictures. In HEVC, the merge candidates list is constructed based on: upto four spatial merge candidates that are derived from five spatialneighboring blocks shown in FIG. 1; one temporal merge candidate derivedfrom two temporal co-located blocks; and additional merge candidatesincluding combined bi-predictive candidates and zero motion vectorcandidates.

In HEVC, a skip mode is used to indicate for a block that the motiondata is inferred instead of explicitly signaled ad that the predictionresidual is zero, i.e. no transform coefficients are transmitted. InHEVC, at the beginning of each CU in an inter-picture prediction slice,a slip _flag is signaled that implies that the CU only contains one PU(2N×2N), that the merge mode is used to derive the motion data, and thatno residual data is present in the bitstream.

In Joint Exploration Model 7 (JEM 7) which is the test model softwarestudy by Joint Video Exploration Team (JVET), some new merge candidatesare introduced. The sub-CU modes are enabled as additional mergecandidates and there is no additional syntax element required to signalthe modes. Two additional merge candidates are added to merge candidateslist of each CU to represent the ATMVP mode and STMVP mode. Up to sevenmerge candidates are used, if the sequence parameter set indicates thatATMVP and STMVP are enabled. The encoding logic of the additional mergecandidates is the same as for the merge candidates in the HEVC, whichmeans, for each CU in P or B slice, two more RD checks are needed forthe two additional merge candidates. In JEM, the order of the insertedmerge candidates is A, B, C, D, ATMVP, STMVP, E (when the mergecandidates in the list are less than 6), TMVP, combined bi-predictivecandidates and zero motion vector candidates.

In the JEM, all bins of merge index are context coded by CABAC. While inHEVC, only the first bin is context coded and the remaining bins arecontext by-pass coded. In the JEM, the maximum number of mergecandidates are 7.

Another scheme searches the candidate motion vectors from previouslycoded blocks, with a step size of 8×8 block. It defines the nearestspatial neighbors, i.e., immediate top row, left column, and top-rightcorner, as category 1. The outer regions (maximum three 8×8 blocks awayfrom the current block boundary) and the collocated blocks in thepreviously coded frame are classified as category 2. The neighboringblocks that are predicted from different reference frames or are intracoded are pruned from the list. The remaining reference blocks are theneach assigned a weight. The weight is related to the distance to thecurrent block. FIG. 2 illustrate an example of the merge candidates listconstruction in this scheme.

In JVET-J0059, more spatial positions are checked as shown in FIG. 3.The extended spatial positions from 6 to 27 are checked according totheir numerical order after the temporal candidate. In order to save theMV line buffer, all the spatial candidates are restricted within two CTUlines. That is, the spatial candidates beyond the CTU line above thecurrent CTU line are excluded. The grid of these extra spatial mergecandidate is based on the block size. Thus every candidate is an offsetof width away from the next one in horizontal direction, and an offsetof height away from the next one in vertical direction. Width and heightare the current block size.

The number of candidates within the merge candidate list is controlledby NumMrgCands. In HEVC, NumMrgCands=5. In JEM, the NumMrgCands is addedby 2 when ATMVP is turned on. In this proposed method, the NumMrgCandsis added by 6, so the NumMrgCands equals to 11 when ATMVP is off and 13when ATMVP is on.

The whole process of adding candidates will stop as soon as the numberof candidates reaches NumMrgCands during the merge candidate listconstruction process. The redundancy check is done for all the mergecandidates except the generated ones. That is, only unique motioncandidate from spatial positions, ATMVP, and temporal candidate, can beincluded the merge candidate list. The merge candidate list isconstructed by adding the spatial positions from position 1 to 5, ATMVPcandidates, and temporal candidate, which is the same as in current JEM(including redundancy check), and adding the extended spatial positionsfrom position 6 to 27 (including redundancy check).

SUMMARY

In an embodiment, there is provided a method for searching for mergecandidates for inter-prediction coding of a video sequence, includingperforming a comparison between a size of a current CU and a thresholdsize, based on a result of the comparison, changing a size of a searchgrid used to search for the merge candidates in order to construct amerge candidate list.

In an embodiment, there is provided a device for searching for mergecandidates for inter-prediction coding of a video sequence including atleast one memory configured to store program code, and at least oneprocessor configured to read the program code and operate as instructedby the program code, the program code including comparison code forperforming a comparison between a size of a current CU and a thresholdsize, and searching code for changing, based on a result of thecomparison, a size of a search grid used to search for the mergecandidates in order to construct a merge candidate list.

In an embodiment, there is provided a non-transitory computer-readablemedium storing instructions, the instructions including one or moreinstructions that, when executed by one or more processors of a devicefor searching for merge candidates for inter-prediction coding of avideo sequence, cause the one or more processors to perform a comparisonbetween a size of a current CU and a threshold size, and based on aresult of the comparison, change a size of a search grid used to searchfor the merge candidates in order to construct a merge candidate list

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a diagram of spatial merge candidates.

FIG. 2 is a diagram showing an example of merge candidate listconstruction.

FIG. 3 is a diagram showing positions of extended spatial candidates.

FIG. 4 is a flowchart of an example process for signaling a transformtype used to encode a current block in an encoded video bitstream.

FIG. 5 is a simplified block diagram of a communication system accordingto an embodiment of the present disclosure.

FIG. 6 is a diagram of the placement of a video encoder and decoder in astreaming environment.

FIG. 7 is a functional block diagram of a video decoder according to anembodiment of the present disclosure.

FIG. 8 is a diagram of an example of a search pattern, according to anembodiment of the present disclosure.

FIG. 9 is a functional block diagram of a video encoder according to anembodiment of the present disclosure.

FIG. 10 is a diagram of a computer system in accordance with anembodiment.

DETAILED DESCRIPTION

FIG. 4 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) may include at least two terminals (410-420)interconnected via a network (450). For unidirectional transmission ofdata, a first terminal (410) may code video data at a local location fortransmission to the other terminal (420) via the network (450). Thesecond terminal (420) may receive the coded video data of the otherterminal from the network (450), decode the coded data and display therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

FIG. 4 illustrates a second pair of terminals (430, 440) provided tosupport bidirectional transmission of coded video that may occur, forexample, during videoconferencing. For bidirectional transmission ofdata, each terminal (430, 440) may code video data captured at a locallocation for transmission to the other terminal via the network (450).Each terminal (430, 440) also may receive the coded video datatransmitted by the other terminal, may decode the coded data and maydisplay the recovered video data at a local display device.

In FIG. 4, the terminals (410-440) may be illustrated as servers,personal computers and smart phones but the principles of the presentdisclosure are not so limited. Embodiments of the present disclosurefind application with laptop computers, tablet computers, media playersand/or dedicated video conferencing equipment. The network (450)represents any number of networks that convey coded video data among theterminals (410-440), including for example wireline and/or wirelesscommunication networks. The communication network (450) may exchangedata in circuit-switched and/or packet-switched channels. Representativenetworks include telecommunications networks, local area networks, widearea networks and/or the Internet. For the purposes of the presentdiscussion, the architecture and topology of the network (450) may beimmaterial to the operation of the present disclosure unless explainedherein below.

FIG. 5 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and decoder in astreaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (513), that caninclude a video source (501), for example a digital camera, creating,for example, an uncompressed video sample stream (502). That samplestream (502), depicted as a bold line to emphasize a high data volumewhen compared to encoded video bitstreams, can be processed by anencoder (503) coupled to the camera 501). The encoder (503) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video bitstream (504), depicted as a thin line toemphasize the lower data volume when compared to the sample stream, canbe stored on a streaming server (505) for future use. One or morestreaming clients (506, 508) can access the streaming server (505) toretrieve copies (507, 509) of the encoded video bitstream (504). Aclient (506) can include a video decoder (510) which decodes theincoming copy of the encoded video bitstream (507) and creates anoutgoing video sample stream (511) that can be rendered on a display(512) or other rendering device (not depicted). In some streamingsystems, the video bitstreams (504, 507, 509) can be encoded accordingto certain video coding/compression standards. Examples of thosestandards include ITU-T Recommendation H.265. Under development is avideo coding standard informally known as Versatile Video Coding (VVC).The disclosed subject matter may be used in the context of VVC.

FIG. 6 may be a functional block diagram of a video decoder (510)according to an embodiment of the present invention.

A receiver (610) may receive one or more codec video sequences to bedecoded by the decoder (510); in the same or another embodiment, onecoded video sequence at a time, where the decoding of each coded videosequence is independent from other coded video sequences. The codedvideo sequence may be received from a channel (612), which may be ahardware/software link to a storage device which stores the encodedvideo data. The receiver (610) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (610) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (615) may be coupled inbetween receiver (610) and entropy decoder/parser (620) (“parser”henceforth). When receiver (610) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosychronous network, the buffer (615) may not be needed, or can besmall. For use on best effort packet networks such as the Internet, thebuffer (615) may be required, can be comparatively large and canadvantageously of adaptive size.

The video decoder (510) may include a parser (620) to reconstructsymbols (621) from the entropy coded video sequence. Categories of thosesymbols include information used to manage operation of the decoder(510), and potentially information to control a rendering device such asa display (512) that is not an integral part of the decoder but can becoupled to it, as was shown in FIG. 6. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (620) mayparse/entropy-decode the coded video sequence received. The coding ofthe coded video sequence can be in accordance with a video codingtechnology or standard, and can follow principles well known to a personskilled in the art, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (620) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameters corresponding to thegroup. Subgroups can include Groups of Pictures (GOPs), pictures, tiles,slices, macroblocks, Coding Units (CUs), blocks, Transform Units (TUs),Prediction Units (PUs) and so forth. The entropy decoder/parser may alsoextract from the coded video sequence information such as transformcoefficients, quantizer parameter (QP) values, motion vectors, and soforth.

The parser (620) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer (615), so to create symbols(621). The parser (620) may receive encoded data, and selectively decodeparticular symbols (621). Further, the parser (620) may determinewhether the particular symbols (621) are to be provided to a MotionCompensation Prediction unit (653), a scaler/inverse transform unit(651), an Intra Prediction Unit (652), or a loop filter (656).

Reconstruction of the symbols (621) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (620). The flow of such subgroup control information between theparser (620) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, decoder (510) can beconceptually subdivided into a number of functional units as describedbelow. In a practical implementation operating under commercialconstraints, many of these units interact closely with each other andcan, at least partly, be integrated into each other. However, for thepurpose of describing the disclosed subject matter, the conceptualsubdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (651). Thescaler/inverse transform unit (651) receives quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (621) from the parser (620). It can output blockscomprising sample values, that can be input into aggregator (655).

In some cases, the output samples of the scaler/inverse transform (651)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (652). In some cases, the intra pictureprediction unit (652) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current (partly reconstructed) picture(656). The aggregator (655), in some cases, adds, on a per sample basis,the prediction information the intra prediction unit (652) has generatedto the output sample information as provided by the scaler/inversetransform unit (651).

In other cases, the output samples of the scaler/inverse transform unit(651) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a Motion Compensation Prediction unit (653) canaccess reference picture memory (657) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (621) pertaining to the block, these samples can beadded by the aggregator (655) to the output of the scaler/inversetransform unit (in this case called the residual samples or residualsignal) so to generate output sample information. The addresses withinthe reference picture memory form where the motion compensation unitfetches prediction samples can be controlled by motion vectors,available to the motion compensation unit in the form of symbols (621)that can have, for example X, Y, and reference picture components.Motion compensation also can include interpolation of sample values asfetched from the reference picture memory when sub-sample exact motionvectors are in use, motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (655) can be subject to variousloop filtering techniques in the loop filter unit (656). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video bitstream andmade available to the loop filter unit (656) as symbols (621) from theparser (620), but can also be responsive to meta-information obtainedduring the decoding of previous (in decoding order) parts of the codedpicture or coded video sequence, as well as responsive to previouslyreconstructed and loop-filtered sample values.

The output of the loop filter unit (656) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (656) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. Once a coded picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, parser (620)), the current reference picture(656) can become part of the reference picture buffer (657), and a freshcurrent picture memory can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology that may be documented in astandard, such as ITU-T Rec. H.265. The coded video sequence may conformto a syntax specified by the video compression technology or standardbeing used, in the sense that it adheres to the syntax of the videocompression technology or standard, as specified in the videocompression technology document or standard and specifically in theprofiles document therein. Also necessary for compliance can be that thecomplexity of the coded video sequence is within bounds as defined bythe level of the video compression technology or standard. In somecases, levels restrict the maximum picture size, maximum frame rate,maximum reconstruction sample rate (measured in, for example megasamplesper second), maximum reference picture size, and so on. Limits set bylevels can, in some cases, be further restricted through HypotheticalReference Decoder (HRD) specifications and metadata for HRD buffermanagement signaled in the coded video sequence.

In an embodiment, the receiver (610) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal-to-noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 7 may be a functional block diagram of a video encoder (503)according to an embodiment of the present disclosure.

The encoder (503) may receive video samples from a video source (501)(that is not part of the encoder) that may capture video image(s) to becoded by the encoder (503).

The video source (501) may provide the source video sequence to be codedby the encoder (503) in the form of a digital video sample stream thatcan be of any suitable bit depth (for example: 8 bit, 10 bit, 12 bit, .. . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ) and anysuitable sampling structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). Ina media serving system, the video source (501) may be a storage devicestoring previously prepared video. In a videoconferencing system, thevideo source (503) may be a camera that captures local image informationas a video sequence. Video data may be provided as a plurality ofindividual pictures that impart motion when viewed in sequence. Thepictures themselves may be organized as a spatial array of pixels,wherein each pixel can comprise one or more samples depending on thesampling structure, color space, etc. in use. A person skilled in theart can readily understand the relationship between pixels and samples.The description below focuses on samples.

According to an embodiment, the encoder (503) may code and compress thepictures of the source video sequence into a coded video sequence (743)in real time or under any other time constraints as required by theapplication. Enforcing appropriate coding speed is one function ofController (750). Controller controls other functional units asdescribed below and is functionally coupled to these units. The couplingis not depicted for clarity. Parameters set by controller can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. A person skilled in the art can readily identify other functionsof controller (750) as they may pertain to video encoder (503) optimizedfor a certain system design.

Some video encoders operate in what a person skilled in the art readilyrecognizes as a “coding loop.” As an oversimplified description, acoding loop can consist of the encoding part of an encoder (730)(“source coder” henceforth) (responsible for creating symbols based onan input picture to be coded, and a reference picture(s)), and a (local)decoder (733) embedded in the encoder (503) that reconstructs thesymbols to create the sample data that a (remote) decoder also wouldcreate (as any compression between symbols and coded video bitstream islossless in the video compression technologies considered in thedisclosed subject matter). That reconstructed sample stream is input tothe reference picture memory (734). As the decoding of a symbol streamleads to bit-exact results independent of decoder location (local orremote), the reference picture buffer content is also bit exact betweenlocal encoder and remote encoder. In other words, the prediction part ofan encoder “sees” as reference picture samples exactly the same samplevalues as a decoder would “see” when using prediction during decoding.This fundamental principle of reference picture synchronicity (andresulting drift, if synchronicity cannot be maintained, for examplebecause of channel errors) is well known to a person skilled in the art.

The operation of the “local” decoder (733) can be the same as of a“remote” decoder (510), which has already been described in detail abovein conjunction with FIG. 6. Briefly referring also to FIG. 7, however,as symbols are available and en/decoding of symbols to a coded videosequence by entropy coder (745) and parser (620) can be lossless, theentropy decoding parts of decoder (510), including channel (612),receiver (610), buffer (615), and parser (620) may not be fullyimplemented in local decoder (733).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. The description of encodertechnologies can be abbreviated as they are the inverse of thecomprehensively described decoder technologies. Only in certain areas amore detail description is required and provided below.

As part of its operation, the source coder (730) may perform motioncompensated predictive coding, which codes an input frame predictivelywith reference to one or more previously-coded frames from the videosequence that were designated as “reference frames.” In this manner, thecoding engine (732) codes differences between pixel blocks of an inputframe and pixel blocks of reference frame(s) that may be selected asprediction reference(s) to the input frame.

The local video decoder (733) may decode coded video data of frames thatmay be designated as reference frames, based on symbols created by thesource coder (730). Operations of the coding engine (732) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (733) replicates decodingprocesses that may be performed by the video decoder on reference framesand may cause reconstructed reference frames to be stored in thereference picture cache (734). In this manner, the encoder (503) maystore copies of reconstructed reference frames locally that have commoncontent as the reconstructed reference frames that will be obtained by afar-end video decoder (absent transmission errors).

The predictor (735) may perform prediction searches for the codingengine (732). That is, for a new frame to be coded, the predictor (735)may search the reference picture memory (734) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(735) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (735), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (734).

The controller (750) may manage coding operations of the video coder(730), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (745). The entropy coder translatesthe symbols as generated by the various functional units into a codedvideo sequence, by loss-less compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (740) may buffer the coded video sequence(s) as createdby the entropy coder (745) to prepare it for transmission via acommunication channel (760), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(740) may merge coded video data from the video coder (730) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (750) may manage operation of the encoder (503). Duringcoding, the controller (750) may assign to each coded picture a certaincoded picture type, which may affect the coding techniques that may beapplied to the respective picture. For example, pictures often may beassigned as one of the following frame types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other frame in the sequence as a source of prediction.Some video codecs allow for different types of Intra pictures,including, for example Independent Decoder Refresh Pictures. A personskilled in the art is aware of those variants of I pictures and theirrespective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded non-predictively,via spatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codednon-predictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video coder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video coder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (740) may transmit additional datawith the encoded video. The video coder (730) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

Embodiments of the present disclosure include several methods forinter-picture prediction coding, for example relating to merge mode. Inembodiments, a merge candidate can come from neighboring CUs that arenot immediately next to the current CU. This may be referred to asextended merge mode. When an encoder and decoder try search for mergecandidates from neighboring CU, the grid size can depend on the CU size.In this case, the smaller CU will have smaller grid size and the largerCU will have larger grid size. These embodiments can be easily extendedto any video coding method that use the merge concept. For example,because skip mode will use merge mode to derive motion information,embodiments described herein can also apply to skip mode.

In an embodiment, when the current CU size is above a threshold, thesearch grid is a fixed searching grid, such as N×M, for example, 16×16.When the current CU size is below or equal to the threshold, the searchgrid is adaptively changing according to the blocks size. For example,the search grid may be a width of the current CU in the horizontaldirection, and a height of the current CU in the vertical direction. Thethreshold can be signaled in, for example, a Sequence parameter set(SPS), a picture parameter set (PPS), or a slice header. The thresholdcan also be predefined.

When the search grid is adaptively changing according to the block size,the detailed search pattern may be an extension of the current JVET/HEVCscheme. The corresponding A (i, j), B (i, j), C (i, j), D (i, j), E (i,j) candidates in the further neighboring blocks are scanned. In thisembodiment, i and j are the coordinates of the row and column. The scanorder can be from nearest neighboring to the neighboring that are faraway. The scheme is illustrated in FIG. 8. When searching theneighboring block for the candidates, the searching grid is adaptivelychange according to the block size. In the horizontal direction, thesearch grid is based on the block width, so each A(i,j) or D(i, j) has adistance of block width to the next A(i,j) or D(i,j). In the verticaldirection, the search grid is based on the height, so each B(i, j) orC(i, j) has a distance of height to the next B(i, j) or C(i, j). In thediagonal direction, the search grid is based on the block width andheight, each E(i, j) to the next E(i, j) has an offset of (width,height) to the next E(i, j). The search range is defined by (offset xand offset y).

In an embodiment, when the current CU size is below a threshold, thesearch grid is a fixed searching grid, such as N×M, for example is16×16. When the current CU size is greater or equal to the threshold,the search grid is adaptively changing according to the blocks size.Namely, the search grid may be a width of the current CU in thehorizontal direction, and a height of the current CU in the verticaldirection. The threshold can be signaled in, for example, an SPS, a PPS,or a slice header. The threshold can also be predefined.

When the search grid is adaptively changing according to the block size,the detailed search pattern may be the same as the search patterndescribed above.

In an embodiment, the adaptive grid size can also be applied to othersearch patterns. In those search patterns, the grid size to find thenext candidate will be adaptively changing according to the currentblock size. Namely, the search grid may be a width of the current blockin the horizontal direction, and a height of the current block in thevertical direction. In one embodiment, when the current CU size is belowa threshold, the search grid is a fixed searching grid, such as N×M, forexample 16×16. When the current CU size is greater or equal to thethreshold, the search grid is adaptively changing according to theblocks size.

In another embodiment, when the current CU size is greater or equal to athreshold, the search grid is a fixed searching grid, such as N×M, forexample 16×16. When the current CU size is below the threshold, thesearch grid is adaptively changing according to the blocks size.

In the embodiments discussed above, when comparing a current CU sizewith a threshold, various aspects of the CU size may be used in thecomparison. For example, a minimum of the width of the CU and a heightof the CU may be used. In another example, a maximum of the width of theCU and a height of the CU may be used. In yet another example, a sum ofthe width of the CU and a height of the CU may be used.

FIG. 9 is a flowchart is a flowchart of an example process 900 forsearching for merge candidates for inter-prediction coding of a videosequence. In some implementations, one or more process blocks of FIG. 9may be performed by decoder 510. In some implementations, one or moreprocess blocks of FIG. 9 may be performed by another device or a groupof devices separate from or including decoder 510, such as encoder 503.

As shown in FIG. 9, process 900 may include performing a comparisonbetween a size of a current CU and a threshold size (block 910). Asfurther shown in FIG. 9, process 900 may include determining, based on aresult of the comparison, whether to change a size of a search grid(block 920). As further shown in FIG. 9, process 900 may includechanging, based on a result of the comparison, a size of a search gridused to search for the merge candidates in order to construct a mergecandidate list (block 930).

In an embodiment, the size of the current CU may be a larger one of aheight of the current CU and a width of the current CU.

In an embodiment, the size of the current CU may be a smaller one of aheight of the current CU and a width of the current CU.

In an embodiment, the size of the current CU may be a sum of a height ofthe current CU and a width of the current CU.

In an embodiment, process 900 may further include, in response todetermining that the size of the current CU is larger than the thresholdsize, setting the size of the search grid to a fixed size, and inresponse to determining that the size of the current CU is smaller thanthe threshold size, setting the size of the search grid to a variablesize based on the size of the current CU.

In an embodiment, process 900 may further include, in response todetermining that the size of the current CU is smaller than thethreshold size, setting the size of the search grid to a fixed size, andin response to determining that the size of the current CU is largerthan the threshold size, setting the size of the search grid to avariable size based on the size of the current CU.

In an embodiment, a horizontal size of the search grid may be determinedbased on a width of the current CU, and a vertical size of the searchgrid may be determined based on a height of the current CU.

In an embodiment, the threshold size may be signaled in at least onefrom among a sequence parameter set, a picture parameter set, and aslice header.

In an embodiment, a scan order used to search for the merge candidateswithin the search grid is represented as A(i,j), B(i,j), C(i,j), D(i,j),E(i,j), where i represents a number of horizontal offsets from thecurrent CU, and where j represents a number of vertical offsets from thecurrent CU. As an example A(0,0) may be located at a left side of thecurrent CU, B(0,0) may be located at a top side of the current CU,C(0,0) may be located at a top-right corner of the current CU, D(0,0)may be located at a bottom-left corner of the current CU, and E(0,0) maybe located at a top-left corner of the current CU.

In an embodiment, the merge candidate list may be used in at least onefrom among a merge mode, an extended merge mode, and a skip mode.

Although FIG. 9 shows example blocks of process 900, in someimplementations, process 900 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 9. Additionally, or alternatively, two or more of theblocks of process 900 may be performed in parallel.

Further, the proposed methods may be implemented by processing circuitry(e.g., one or more processors or one or more integrated circuits). Inone example, the one or more processors execute a program that is storedin a non-transitory computer-readable medium to perform one or more ofthe proposed methods.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 10 shows a computersystem 1200 suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by computer central processing units (CPUs),Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 10 for computer system 1200 are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system 1200.

Computer system 1200 may include certain human interface input devices.Such a human interface input device may be responsive to input by one ormore human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard 1001, mouse 1002, trackpad 1003, touch screen1010, data-glove 1204, joystick 1005, microphone 1006, scanner 1007,camera 1008.

Computer system 1200 may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen 1010, data-glove 1204, or joystick 1005, but there can alsobe tactile feedback devices that do not serve as input devices), audiooutput devices (such as: speakers 1009, headphones (not depicted)),visual output devices (such as screens 1010 to include cathode ray tube(CRT) screens, liquid-crystal display (LCD) screens, plasma screens,organic light-emitting diode (OLED) screens, each with or withouttouch-screen input capability, each with or without tactile feedbackcapability—some of which may be capable to output two dimensional visualoutput or more than three dimensional output through means such asstereographic output; virtual-reality glasses (not depicted),holographic displays and smoke tanks (not depicted)), and printers (notdepicted).

Computer system 1200 can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW1020 with CD/DVD or the like media 1021, thumb-drive 1022, removablehard drive or solid state drive 1023, legacy magnetic media such as tapeand floppy disc (not depicted), specialized ROM/ASIC/PLD based devicessuch as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system 1200 can also include interface(s) to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include global systems for mobile communications(GSM), third generation (3G), fourth generation (4G), fifth generation(5G), Long-Term Evolution (LTE), and the like, TV wireline or wirelesswide area digital networks to include cable TV, satellite TV, andterrestrial broadcast TV, vehicular and industrial to include CANBus,and so forth. Certain networks commonly require external networkinterface adapters that attached to certain general purpose data portsor peripheral buses (1049) (such as, for example universal serial bus(USB) ports of the computer system 1200; others are commonly integratedinto the core of the computer system 1200 by attachment to a system busas described below (for example Ethernet interface into a PC computersystem or cellular network interface into a smartphone computer system).Using any of these networks, computer system 1200 can communicate withother entities. Such communication can be uni-directional, receive only(for example, broadcast TV), uni-directional send-only (for exampleCANbus to certain CANbus devices), or bi-directional, for example toother computer systems using local or wide area digital networks.Certain protocols and protocol stacks can be used on each of thosenetworks and network interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core 1040 of thecomputer system 1200.

The core 1040 can include one or more Central Processing Units (CPU)1041, Graphics Processing Units (GPU) 1042, specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)1043, hardware accelerators for certain tasks 1044, and so forth. Thesedevices, along with Read-only memory (ROM) 1045, Random-access memory(RAM) 1046, internal mass storage such as internal non-user accessiblehard drives, solid-state drives (SSDs), and the like 1047, may beconnected through a system bus 1248. In some computer systems, thesystem bus 1248 can be accessible in the form of one or more physicalplugs to enable extensions by additional CPUs, GPU, and the like. Theperipheral devices can be attached either directly to the core's systembus 1248, or through a peripheral bus 1049. Architectures for aperipheral bus include peripheral component interconnect (PCI), USB, andthe like.

CPUs 1041, GPUs 1042, FPGAs 1043, and accelerators 1044 can executecertain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM1045 or RAM 1046. Transitional data can be also be stored in RAM 1046,whereas permanent data can be stored for example, in the internal massstorage 1047. Fast storage and retrieve to any of the memory devices canbe enabled through the use of cache memory, that can be closelyassociated with one or more CPU 1041, GPU 1042, mass storage 1047, ROM1045, RAM 1046, and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture 1200, and specifically the core 1040 can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core 1040 that are of non-transitorynature, such as core-internal mass storage 1047 or ROM 1045. Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core 1040. A computer-readablemedium can include one or more memory devices or chips, according toparticular needs. The software can cause the core 1040 and specificallythe processors therein (including CPU, GPU, FPGA, and the like) toexecute particular processes or particular parts of particular processesdescribed herein, including defining data structures stored in RAM 1046and modifying such data structures according to the processes defined bythe software. In addition or as an alternative, the computer system canprovide functionality as a result of logic hardwired or otherwiseembodied in a circuit (for example: accelerator 1044), which can operatein place of or together with software to execute particular processes orparticular parts of particular processes described herein. Reference tosoftware can encompass logic, and vice versa, where appropriate.Reference to a computer-readable media can encompass a circuit (such asan integrated circuit (IC)) storing software for execution, a circuitembodying logic for execution, or both, where appropriate. The presentdisclosure encompasses any suitable combination of hardware andsoftware.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

The invention claimed is:
 1. A method of searching for merge candidatesfor inter-prediction coding of a video sequence, the method comprising:performing a comparison between an aspect of a current CU and athreshold size; based on a result of the comparison, changing a size ofa search grid used to search for the merge candidates in order toconstruct a merge candidate list, wherein the changing of the size ofthe search grid comprises changing a horizontal size of the search gridto be a width of the current CU, and changing a vertical size of thesearch grid to be a height of the current CU, wherein, in response todetermining that the aspect of the current CU is larger than thethreshold size, the size of the search grid is set to a fixed size,wherein, in response to determining that the aspect of the current CU issmaller than the threshold size, the size of the search grid is set to avariable size based on the size of the current CU, and wherein thesearch grid is searched in a search pattern related to an extended mergemode.
 2. The method of claim 1, wherein the aspect of the current CUcomprises a larger one of a height of the current CU and a width of thecurrent CU.
 3. The method of claim 1, wherein the aspect of the currentCU comprises a smaller one of a height of the current CU and a width ofthe current CU.
 4. The method of claim 1, wherein the aspect of thecurrent CU comprises a sum of a height of the current CU and a width ofthe current CU.
 5. The method of claim 1, wherein the threshold size issignaled in at least one from among a sequence parameter set, a pictureparameter set, and a slice header.
 6. The method of claim 1, wherein ascan order used to search for the merge candidates within the searchgrid is represented as A(i,j), B(i,j), C(i,j), D(i,j), E(i,j), wherein irepresents a number of horizontal offsets from the current CU, wherein jrepresents a number of vertical offsets from the current CU, whereinA(0,0) is located at a left side of the current CU, wherein B(0,0) islocated at a top side of the current CU, wherein C(0,0) is located at atop-right corner of the current CU, wherein D(0,0) is located at abottom-left corner of the current CU, and wherein E(0,0) is located at atop-left corner of the current CU.
 7. The method of claim 1, wherein themerge candidate list is used in at least one from among a merge mode anda skip mode.
 8. A device for searching for merge candidates forinter-prediction coding of a video sequence, the device comprising: atleast one memory configured to store program code; and at least oneprocessor configured to read the program code and operate as instructedby the program code, the program code including: comparison code forperforming a comparison between an aspect of a current CU and athreshold size; searching code for changing, based on a result of thecomparison, a size of a search grid used to search for the mergecandidates in order to construct a merge candidate list, wherein thechanging of the size of the search grid comprises changing a horizontalsize of the search grid to be a width of the current CU, and changing avertical size of the search grid to be a height of the current CU,wherein, in response to determining that the aspect of the current CU islarger than the threshold size, the size of the search grid is set to afixed size, wherein, in response to determining that the aspect of thecurrent CU is smaller than the threshold size, the size of the searchgrid is set to a variable size based on the size of the current CU, andwherein the search grid is searched in a search pattern related to anextended merge mode.
 9. The device of claim 8, wherein the aspect of thecurrent CU comprises a larger one of a height of the current CU and awidth of the current CU.
 10. The device of claim 8, wherein the aspectof the current CU comprises a smaller one of a height of the current CUand a width of the current CU.
 11. The device of claim 8, wherein theaspect of the current CU comprises a sum of a height of the current CUand a width of the current CU.
 12. The device of claim 8, wherein themerge candidate list is used in at least one from among a merge mode anda skip mode.
 13. A non-transitory computer-readable medium storinginstructions, the instructions comprising: one or more instructionsthat, when executed by one or more processors of a device for searchingfor merge candidates for inter-prediction coding of a video sequence,cause the one or more processors to: perform a comparison between anaspect of a current CU and a threshold size; based on a result of thecomparison, change a size of a search grid used to search for the mergecandidates in order to construct a merge candidate list, wherein thechanging of the size of the search grid comprises changing a horizontalsize of the search grid to be a width of the current CU, and changing avertical size of the search grid to be a height of the current CU,wherein, in response to determining that the aspect of the current CU islarger than the threshold size, the size of the search grid is set to afixed size, wherein, in response to determining that the aspect of thecurrent CU is smaller than the threshold size, the size of the searchgrid is set to a variable size based on the size of the current CU, andwherein the search grid is searched in a search pattern related to anextended merge mode.